Author Bio

Michael Carano is with RPB Chemical Technology. He has been involved in the PWB, general metal finishing photovoltaic industries for nearly 30 years and is currently co-chair of the IPC Technology Roadmap Executive Committee. Carano holds nine U.S. patents in topics including plating, metallization processes and PWB fabrication techniques. He was inducted into the IPC Hall of Fame in 2014 and has been a regular contributor to various industry publications throughout his career.

CASE STUDY: Pits and Mouse Bite Issues, Part 2

Introduction

In last month’s column, I introduced a case study that centered on plating pits and mouse bites. There were three areas in the process that raised concern as to the potential root cause of the defect. Of course, as with the case in all troubleshooting situations, it is best to look at the problem with wide open eyes. Just because one is looking at an issue that is visible after copper plating, this should not mean that is the only place to look. And this case study illustrates that point quite clearly.

The Problem

Again, keep in mind that there seems to be two very different types of plating defects. One particular pit seems to be random, and that there is no pattern that can be detected (Figure 1). In this case, these pits can be seen practically anywhere on the board. In addition, the characteristic of this type of pit essentially is found to go down to the base metal. In Figure 1, the reader can easily see that the plated copper is evident around the pitted area. The pit itself can be described as irregular in shape. Obviously not the same shape as a pit caused by an air bubble.

There can be several possibilities here. One issue may relate to organic contamination of the plating solution itself. Or, is it possible that the primary photoresist was improperly exposed and that allowed resist materials to leach onto the copper surface? In general, interactions between the photoresist and plating process are quite subtle.

Resists have been associated with a variety of plating failures. These failures increase in severity as the technology shifts to higher circuit densities and smaller vias and pads. There are some reasonably clear ties between plating problems and photoresist, as in the case of resist development residues causing copper-to-copper peelers, or stripping residues causing ragged plated lines after etching. One should ask if the pre-plate cleaning is effectively cleaning the copper surface (the area that is exposed after developing) prior to the electrodeposition of copper.

Now, remember the fabricator is also dealing with another type of pit that is more accurately described as a mouse bite. This defect is seen at the resist sidewall/copper interface. These pits are more spherical in shape and do not always lead to the base metal.

An inspection of the copper plating cells provided some additional information that should be noted: All the copper plating cells in the facility utilize air sparger agitation. An observation by a member of the tech team indicated that small air or gas bubbles in the copper plating cells were a potential cause of the spherical pits along the edges of the traces and pads. Further, the team felt that the air agitation created very small air or gas bubbles that would lodge onto the resist sidewall leading to the spherical pits. The agitation system for these plating cells required further scrutiny.

To test the theory that air bubbles could be causing the defect, the fabricator sent panels to another fabricator for processing through their pattern plating line. The agitation for the copper cells was provided by eductors, not air spargers. (Eductors move the plating solution in a more uniform way and eliminate air bubbles.)

After the panels were plated, inspectors noted that there was no evidence of pits or mouse bites along the resist/trace interface. However, there were still some random pits that indicated that residue on the base metal was inhibiting plating. This situation indicated two things: (1) air or gas bubbles at the fabricator were playing a role in the mouse bite defects and (2) there was a residue or contamination that led to the irregularly shaped pits (the subject of next month’s column).

Finally, another critical observation provided further insight to the mouse bite defect. Here one engineer noticed that plating solution was entering the filtration housing in two of the cells (Figure 2).

As this solution entered the housing, the air bubbles would become homogenized, so to speak, allowing the plating solution to become supersaturated with air bubbles. This situation allowed the bubbles to grow and eventually lodge onto the resist sidewall/trace interface(1). To prevent the air bubbles from entering the filtration housing, a system of baffles was installed. The baffles will prevent air bubbles from entering the filtration housing. Of course, a much wiser choice is to convert the agitation system to eductors.

Conclusion

The dominant cause of spherical or conical pits in copper plating is gas or air bubbles, which can be introduced into the plating solution from air spargers or through supersaturation of the plating solution with air bubbles. These bubbles grow on the copper traces at the photoresist sidewall, and thus inhibit plating. Some photoresists may contribute to air bubble (mouse bite) pits depending on the resist’s wetting characteristics, degree of hydrophobicity, and surface imperfections within the resist itself. In Part 3 of this case study, the development process will be reviewed and any possible parameters that may contribute to the pitting defects.

2018

In last month’s column, I introduced a case study that centered on plating pits and mouse bites. There were three areas in the process that raised concern as to the potential root cause of the defect. Of course, as with the case in all troubleshooting situations, it is best to look at the problem with wide open eyes. Just because one is looking at an issue that is visible after copper plating, this should not mean that is the only place to look. And this case study illustrates that point quite clearly.

2017

Most of the dielectric materials that are used to make printed circuit boards incorporate reinforcement into the resin system. Reinforcement usually takes the form of woven glass fiber. Woven fiberglass is just like any other cloth, made up of individual filaments that are woven together on a loom.

In Part 1 of this series on the importance of rinsing, the author presented an overview of the critical aspects of rinsing as it applies to the overall quality of a printed circuit board, with considerable space devoted to water conservation. Thus, we now turn to how one can improve rinsing effectiveness without increasing water consumption and, by default, significant waste treatment costs.

In the first part of a series of columns focused on microvias, the importance of adopting HDI technology as a strategic initiative for the PWB fabricator is presented. The major drivers for HDI are listed.

In nearly every chemical process step in the PCB industry, rinsing is an immediate and required process step. Rinsing is typically a crucial step following a chemical process, and is thought to be one that requires little or no attention to function properly. However, problems caused by ineffective rinsing are responsible for many rejects, as well as huge operating costs in the waste treatment department.

Inadequate or excessive desmear will lead to several PTH defects and failures. Resin smear, ineffective texturing of the resin, and even overly aggressive desmear will contribute to poor plating, adhesion failures and a myriad of other non-conforming defects. However, proper troubleshooting protocol dictates that the engineer also looks at drilling as the contributor to these and other defects.

Follow good shop practices in terms of surface preparation, electroless copper plating thickness and resist developing parameters. Optimal surface preparation utilizing cleaners and micro-etches is critical to eliminating copperto-copper peeling. In addition, over- and underdeveloping create their own set of process constraints. Pay very close attention to developer pH, operating temperature and break point

Metalizing materials such as polyimide used for flexible circuitry provides a significant challenge for process engineers. Conventional electroless copper systems often required pre-treatments with hazardous chemicals or have a small process window to achieve a uniform coverage without blistering.

Electroplating a printed circuit board is by no means a trivial task. Higher layer counts, smaller-diameter vias (through-hole and blind) as well as higher-performance material sets contribute to the greater degree of difficulty with today’s technology.

2016

In this month’s installment of Trouble in Your Tank, I will further explore the critical drilling parameters required to drill a “good hole” and provide information on some little-known parameters required for this operation.

In the extensive process of printed circuit board fabrication, one of the steps involves mechanically drilling through-hole vias. Via formation is then followed by desmear and metallization. The quality of the through-hole drilling process (and by inference the quality of the drilled through-hole) or lack thereof will also impact the desmear and metalization processes.

Introduction Sometimes the problems can really get pretty ugly. One would assume these uglies would be easy to correct. But most often, solving such a problem requires a much deeper dig into the process.

In Part 1 of this column on reliability, I presented the common PTH failures encountered when reliability is less than robust. PTH reliability is influenced by several factors including the quality of the PTH after drilling, plating thickness and plating distribution in the PTH. In this column, I will present additional factors, including the Coffin-Manson model in the context of understanding reliability failures.

Sometimes there is confusion among PCB engineers and quality managers as to what constitutes reliability. Some may say that reliability refers to avoiding PTH failures such as corner cracks or interconnect defects. Then there are those who subscribe to a wider range of failure criteria to determine whether or not the final product is reliable for long-term service.

The proper development of the primary photoresist is critical to the overall success of the imaging process and in turn the processes that follow—either etching to form innerlayers or the electroplating processes on outer layers. In this step, the unexposed photoresist (after resist lamination and exposure) is washed away via the developing process.

It is the job of the PCB process engineer to ensure that a quality circuit is delivered. This process starts with sound mechanics of the im-aging system that include surface cleanliness and resist lamination parameters.

2015

Getting the OSP process to perform as it is intended requires attention to both the equipment operating parameters as well as chemistry. This month's "Trouble in Your Tank" delves into one of the most irritating issues with respect to OSP: discoloration (read "oxidation") on critical circuit features.

Many factors are in play when it comes to preventing ink from remaining in vias. In this article, Michael Carano presents two factors: solder not flowing in the vias and the lack of a plated solderable finish in the vias. This may be because some of the vias have a final finish in them and others do not--and all of these are happening on the same boards!

Half-etch technology is production proven in meeting high density circuitry requirements. Thinner copper is effective in achieving sub-35 micron lines and spaces due to the total copper thickness that must be etched.

In this case study, a PCB fabricator investigates the root cause of solderability defects on ENIG-processed circuits. It was determined that the fabricator, due to process errors, caused hyper-corrosion within the nickel deposit, which led to the defects.

2014

While next-generation OSP as a final finish has become the standard for lead-free compatible assembly, one should assume that any new OSP meets the criteria. A number of simple procedures may be followed to qualify any new OSP and ensure it is compatible with these higher assembly temperatures. Columnist Michael Carano takes a closer look.

The aggressive nature of the ENIG process is a particular nuisance for some aqueous-based LPIs. Simply scrubbing the copper surface prior to soldermask application is often not an effective adhesion promotion mechanism for LPI and ENIG. Before exploring surface topography further, it is important to understand outside influences such as ENIG and its effect on adhesion.

The aggressive nature of the ENIG process is a particular nuisance for some aqueous-based LPIs. Simply scrubbing the copper surface prior to soldermask application is often not an effective adhesion promotion mechanism for LPI and ENIG. Before exploring surface topography further, it is important to understand outside influences such as ENIG and its effect on adhesion.

Understanding the interactions of the materials, oxide treatment, and the lamination process will help you get to the root cause failures in multilayer fabrication. When troubleshooting multilayer defects, it is necessary to understand the effect certain process parameters have on quality and reliability. Truly, the quality of a multilayer PCB (prior to desmear/metallization) will depend on several factors presented in this column.

Understanding the interactions of the materials, oxide treatment, and the lamination process will help you get to the root cause failures in multilayer fabrication. When troubleshooting multilayer defects, it is necessary to understand the effect certain process parameters have on quality and reliability. Truly, the quality of a multilayer PCB (prior to desmear/metallization) will depend on several factors presented in this column.

Ideally, the ENIG process must provide the optimum in solder joint reliability while operating at the highest level of cost efficiency. All too often, process parameters that have the most influence on these critical attributes are poorly understood.

Ideally, the ENIG process must provide the optimum in solder joint reliability while operating at the highest level of cost efficiency. All too often, process parameters that have the most influence on these critical attributes are poorly understood.

Columnist Michael Carano writes, "It is all about optimizing the performance of the oxide alternative chemistry. This includes close monitoring of the main reactive ingredients of the process chemistry. And one of the first issues that the industry had to address, whether one is using reduced oxide chemistry or oxide alternatives, is pink ring."

Columnist Michael Carano presents additional information about nickel hyper-corrosion, a spike or fissure in the nickel deposit evident after immersion gold plating, by further defining the five degrees of hyper-corrosion. He also explores the root causes of such attacks on the base nickel along with strategies to mitigate these effects.

The basic fundamentals of PTH drilling revolve around several key factors: 1) speeds and feeds--drill in-feed rate and spindle speed of the drill bit; 2) surface feet per minute; and 3) the material to be drilled. Understanding and applying these first few critical factors will influence the overall quality of the drilled, plated through-hole.

2013

Circuit designs with three-mil lines and spaces are increasingly becoming the norm. Optimizing the imaging process should be of paramount concern. Over the next few months, Mike Carano will present the critical steps in the imaging process and provide insight as to where potential yield reducing defects can occur and how to prevent them.

In Part 1 and 2 of this series, Columnist Michael Carano presented critical information on ink properties, methods of soldermask application, tack drying, and more. In Part 3, the exposure process is detailed, including exposure units, UV lamps, the phototool, and overall exposure energy.

In past columns, Michael Carano presented information about the different types of ICD interconnect defect and its root causes. One key defect that was not discussed is the infamous D-sep. What exactly is D-sep? Carano will explore this issue and provide suggestions for process improvement so you don't experience D-sep on your expensive, high-reliability PCBs.

Achieving a void-free deposit through the PTH line need not be an elusive goal. Instead, careful thought should be given to line design and the critical interactions of chemical parameters. In addition, the fabricator and supplier must work closely to ensure high-performance materials can be processed with the highest levels of reliability.

Both a reduced oxide process and an alternative oxide enhance the bond strength between the prepreg resin and the copper. In addition, the treated foil, if exposed by the wedge, is better able to stave off dissolution from the many acidic process steps to which the PWB will be subjected.

The soldermask is an integral part of the circuit board. The proper application of soldermask requires strict attention to detail. This first article of a two-part series elucidates the critical operational requirements of the photoimageable soldermask process.

Negative etchback is not a non-conforming defect according to IPC-600 (within limits). Acceptable levels of negative etchback do exist; however, there is a downside that can affect the overall plating quality in the PTH.

This case study from Michael Carano emphasizes the critical thinking required to solve complex technical issues related to innerconnect defect. The defect described relates to drilling and inadequate drill smear removal, as opposed to plating as the root cause of the innerplane separation.

2012

Soldermask can peel or lift off areas on a PCB for a variety of reasons. This edition of "Trouble in Your Tank" presents the causes of soldermask peeling and lifting and stresses the importance of surface preparation prior to mask application.

2011

In previous columns related to the electroless nickel-immersion gold process, Michael Carano focused on plating and process defects related to mostly pre-plate process steps such as poor or inadequate cleaning, micro-etching and rinsing. This month, he tackles additional annoyances that lead to scrap and reduce the confidence in the ENIG process.